Scientists from two national labs used Argonne’s Advanced Photon Source to answer a decades-old question.
For computer semiconductors and modern communications equipment, finding more efficient ways to conduct an electrical charge opens up exciting possibilities. Devices can be smaller, perform digital tasks faster and consume less energy.
To make this possible, physicists had to figure out what happens electronically when a material transitions between states, from one that doesn’t conduct electricity to one that does. That question has haunted scientists for 60 years, but a recent groundbreaking discovery involving physicists from two national labs has lifted the veil on the answer.
“The RIXS beamline is one of the best in the world for this experiment. The technique was really crucial.” — Gilberto Fabbris, Argonne National Laboratory
“Physicists were trying to figure out what happens when you narrow the electronic ‘energy gap’ between an insulator and a conductor,” said Daniel Mazzone, a former physicist at Brookhaven National Laboratory, now at the Paul Scherrer Institut in Switzerland. a recent study of demand. Mazzone and his colleagues, which include physicists from both Brookhaven and Argonne National Laboratory, published their results in February in nature communication† “Do you just turn a simple insulator into a simple metal where the electrons can move freely, or does something more interesting happen?”
Using the Advanced Photon Source (APS), a user facility of the Department of Energy (DOE) Office of Science at DOE’s Argonne National Laboratory, a team of Brookhaven researchers collaborated with their Argonne colleagues to show what previous scientists had only speculated. Under certain circumstances, a new magnetic state of matter can be found in which magnetic moments of electrons (also called “spins”) are closely related to their insulating property. This new magnetic state is called the antiferromagnetic excitonic insulator state.
“An insulator is the opposite of a metal; it’s a material that doesn’t conduct electricity,” said Brookhaven scientist Mark Dean, a co-author of the paper. “The electrons are all stuck in place, like people in a filled amphitheatre; they cannot move.”
To get the electrons out of this low or “ground” energy state, they need a big energy boost. The boost must be large enough to bridge a gap between the ground state and a higher energy level. In very special circumstances, an energy gain can outweigh the energy costs of electrons jumping across the energy gap.
“The energy gap between an insulator and a conductor can be somewhat compared to the ditch between a two-lane highway,” said Argonne physicist Mary Upton, a co-author of the paper. “Imagine that one lane is completely filled with cars and the other lane is completely open. Filling the ditch allows cars to circulate freely and continue to drive. In an antiferromagnetic insulator state, electrons can do the same.”
In their experiment, the collaborative team worked with a material called strontium iridium oxide, which insulates only barely above room temperature.
“The sample itself already had a very small energy gap, but what we were trying to figure out was whether the sample was a normal insulator or if it was already a magnetic excitonic insulator,” said Argonne assistant physicist Gilberto Fabbris, a co-author on the paper. “A magnetic excitonic insulator has the energy electrons needed to move, but a special type of magnetic interaction blocks them, meaning the material doesn’t conduct electricity.” Without doing anything but changing the temperature, the team showed that the material was indeed a magnetic excitonic insulator.
Advanced techniques and instruments made it possible for the team to study this phenomenon. For example, the Resonant Inelastic X-Ray Scattering (RIXS) beamline from the APS on 27-ID helped the scientists look deep into the material. RIXS uses X-rays to measure how much energy and momentum is lost when light causes an excitation (a type of disturbance) of the electrons in a material. It enabled Brookhaven and Argonne’s team to measure magnetic interactions and the associated energy costs of moving electrons.
Upton and Fabbris said a facility like the APS, which generates exponentially brighter X-rays than smaller, lab-based machines, is essential for making this measurement.
“The RIXS beamline is one of the best in the world for this experiment,” Fabbris said, “The technique was really crucial.”
The APS is currently undergoing a massive upgrade that will increase the brightness of its X-rays by up to 500 times. As part of that upgrade, a new RIXS instrument was installed on 27-ID, expanding the capabilities of the beamline.
The identification of the antiferromagnetic excitonic insulator completes a long journey into the fascinating ways electrons arrange themselves in materials. In the future, understanding the connections between spin and charge in such materials could have potential for realizing new technologies.
A version of this release was originally posted by Brookhaven National Laboratory†
About the Advanced Photon Source
The US Department of Energy Office of Science’s Advanced Photon Source (APS) at Argonne National Laboratory is one of the world’s most productive X-ray light source facilities. The APS provides ultra-bright X-rays to a diverse community of researchers in materials science, chemistry, condensed matter physics, life and environmental sciences, and applied research. These X-rays are ideally suited for explorations of materials and biological structures; elemental distribution; chemical, magnetic, electronic states; and a wide range of technologically important engineering systems from: batteries to fuel injector sprays, all of which are the foundation of our nation’s economic, technological and physical well-being. Each year, more than 5,000 researchers use the APS to produce more than 2,000 publications detailing important discoveries and solving more vital biological protein structures than users of any other X-ray light source research facility. APS scientists and engineers are innovating technology at the heart of the advancing operation of accelerators and light sources. This includes the insertion devices that produce extremely bright X-rays that are valued by researchers, lenses that focus the X-rays down to a few nanometers, instrumentation that maximizes the way the X-rays interact with samples being studied, and software that collects and manages the massive amount of data arising from discovery research at the APS.
This study used resources from the Advanced Photon Source, a US DOE Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory under contract number DE-AC02-06CH11357.
National Laboratory Argonne seeks solutions to pressing national problems in science and technology. Argonne, the country’s first national laboratory, conducts industry-leading basic and applied scientific research in virtually every scientific discipline. Argonne’s researchers work closely with researchers from hundreds of companies, universities, and federal, state and municipal agencies to help them solve their specific problems, advance America’s scientific leadership, and prepare the nation for a better future. future. With employees from more than 60 countries, Argonne is led by: UChicago Argonne, LLC for the Office of Science of the United States Department of Energy†
The Office of Science of the United States Department of Energy is the largest proponent of basic research in the natural sciences in the United States, working to address some of the most pressing challenges of our time. For more information visit https://energy.gov/science†
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